1. Field of the Invention
The invention relates to a method of determining a nuclear magnetization distribution in a region of an object in which a stimulated echo is generated and sampled by means of a sequence of four rf pulses in combination with different gradient magnetic fields.
2. Description of the Related Art
A method of this kind can be used for spectroscopy measurements and also for imaging measurements where zoom techniques can also be applied when use is made of the selective excitation pulses, as described in a publication by Frahm et al, Journal of Magnetic Resonance, 64, pages 81-93, 1985. Using such a pulse sequence, a small sub-volume of the object can be selected for spectroscopic or imaging measurements as described in European Patent Application 0.184.840 (corresponding to U.S. Pat. No. 4,748,409) where each time a different gradient magnetic field is activated for each 90.degree. pulse of the three-pulse sequence. Using the described method a high-resolution image can be made of a small sub-volume. However, if spectroscopy is to be performed, the described method is very suitable for selecting a small volume, but imposes the problem that this sequence is not suitable for use with a water suppression method. Water suppression is necessary for the measurement of, for example, metabolites in proton spectra. The suppression of the water peak is necessary because the water protons represent the vast majority of the excited nuclear spins.
Therefore, European Patent Application 0.184.840 proposes the application of a 180.degree. refocusing pulse after the three spatially selective 90.degree. pulses. By suitably choosing the intervals between the successive first and second 90.degree. pulse, between the second and the third 90.degree. pulse, and between the third 90.degree. pulse and the 180.degree. refocusing pulse, the water peak can be suppressed as described with reference to FIG. 36 of the cited Application.
However, the proposed pulse sequence leads to a mediocre suppression only of the undesirable component, because suppression is based on differences in relaxation time constants (T1 and T2 relaxation time constant). It is also to be noted that the non-selective 180.degree. pulse will never be ideal and causes interference signals which have a substantial adverse effect on the quality of the echo resonance signal formed.
It is a further drawback of the pulse sequences described in European Patent Application 0.184.840 that they cannot be used for the examination of materials having a comparatively short T2 relaxation time constant.